Wnts are encoded by a large gene family whose members have been found in round worms, insects, cartilaginous fish, and vertebrates. Holland et al., Dev. Suppl., 125-133 (1994). Wnts are thought to function in a variety of developmental and physiological processes since many diverse species have multiple conserved Wnt genes. McMahon, Trends Genet., 8: 236-242 (1992); Nusse and Varmus, Cell, 69: 1073-1087 (1992). Wnt genes encode secreted glycoproteins that are thought to function as paracrine or autocrine signals active in several primitive cell types. McMahon, supra; Nusse and Varmus, supra. The Wnt growth factor family includes more than ten genes identified in the mouse (Wnt-1, -2, -3A, -3B, -4, -5A, -5B, -6, -7A, -7B, -8A, -8B, -10B, -11, -12, and -13) (see, e.g., Gavin et al., Genes Dev., 4: 2319-2332 (1990); Lee et al., Proc. Natl. Acad. Sci. USA, 92: 2268-2272 (1995); Christiansen et al., Mech. Dev., 51: 341-350 (1995)) and at least nine genes identified in the human (Wnt-1, -2, -3, -5A, -7A, -7B, -8B, -10B, and -11) by cDNA cloning. See, e.g., Vant Veer et al., Mol. Cell. Biol., 4: 2532-2534 (1984).
The Wnt-1 proto-oncogene (int-1) was originally identified from mammary tumors induced by mouse mammary tumor virus (MMTV) due to an insertion of viral DNA sequence. Nusse and Varmus, Cell, 31: 99-109 (1982). In adult mice, the expression level of Wnt-1 mRNA is detected only in the testis during later stages of sperm development. Wnt-1 protein is about 42 KDa and contains an amino-terminal hydrophobic region, which may function as a signal sequence for secretion. Nusse and Varmus, supra, 1992. The expression of Wnt-2/irp is detected in mouse fetal and adult tissues and its distribution does not overlap with the expression pattern for Wnt-1. Wnt-3 is associated with mouse mammary tumorigenesis. The expression of Wnt-3 in mouse embryos is detected in the neural tubes and in the limb buds. Wnt-5a transcripts are detected in the developing fore- and hind limbs at 9.5 through 14.5 days and highest levels are concentrated in apical ectoderm at the distal tip of limbs. Nusse and Varmus, supra (1992). Recently, a Wnt growth factor, termed Wnt-x, was described (WO95/17416) along with the detection of Wnt-x expression in bone tissues and in bone-derived cells. Also described was the role of Wnt-x in the maintenance of mature osteoblasts and the use of the Wnt-x growth factor as a therapeutic agent or in the development of other therapeutic agents to treat bone-related diseases.
Wnts may play a role in local cell signaling. Biochemical studies have shown that much of the secreted Wnt protein can be found associated with the cell surface or extracellular matrix rather than freely diffusible in the medium. Papkoff and Schryver, Mol. Cell. Biol., 10: 2723-2730 (1990); Bradley and Brown, EMBO J., 9: 1569-1575 (19:90).
Studies of mutations in Wnt genes have indicated a role for Wnts in growth control and tissue patterning. In Drosophila, wingless (wg) encodes a Wnt-related gene (Rijsewik et al., Cell, 50: 649-657 (1987)) and wg mutations alter the pattern of embryonic ectoderm, neurogenesis, and imaginal disc outgrowth. Morata and Lawerence, Dev. Biol., 56: 227-240 (1977); Baker, Dev. Biol., 125: 96-108 (1988); Klingensmith and Nusse, Dev. Biol., 166: 396-414 (1994). In Caenorhabditis elegans, lin-44 encodes a Wnt homolog which is required for asymmetric cell divisions. Herman and Horvitz, Development, 120: 1035-1047 (1994). Knock-out mutations in mice have shown Wnts to be essential for brain development (McMahon and Bradley, Cell, 62: 1073-1085 (1990); Thomas and Cappechi, Nature, 346: 847-850 (1990)), and the outgrowth of embryonic primordia for kidney (Stark et al., Nature, 372: 679-683 (1994)), tail bud (Takada et al., Genes Dev., 8: 174-189 (1994)), and limb bud. Parr and McMahon, Nature, 374: 350-353 (1995). Overexpression of Wnts in the mammary gland can result in mammary hyperplasia (McMahon, supra (1992); Nusse and Varmus, supra (1992)), and precocious alveolar development. Bradbury et al., Dev. Biol., 170: 553-563 (1995).
Wnt-5a and Wnt-5b are expressed in the posterior and lateral mesoderm and the extraembryonic mesoderm of the day 7-8 murine embryo. Gavin et al., supra. These embryonic domains contribute to the AGM region and yolk sac tissues from which multipotent hematopoietic precursors and HSCs are derived. Dzierzak and Medvinsky, Trends Genet., 11: 359-366 (1995); Zon et al., in Gluckman and Coulombel, ed., Colloque, INSERM, 235: 17-22 (1995), presented at the Joint International Workshop on Foetal and Neonatal Hematopoiesis and Mechanism of Bone Marrow Failure, Paris France, Apr. 3-6, 1995; Kanatsu and Nishikawa, Development, 122: 823-830 (1996). Wnt-5a, Wnt-10b, and other Wnts have been detected in limb buds, indicating possible roles in the development and patterning of the early bone microenvironment as shown for Wnt-7b. Gavin et al., supra; Christiansen et al., Mech. Devel., 51: 341-350 (1995); Parr and McMahon, supra.
The Wnt/Wg signal transduction pathway plays an important role in the biological development of the organism and has been implicated in several human cancers. This pathway also includes the tumor suppressor gene, APC. Mutations in the APC gene are associated with the development of sporadic and inherited forms of human colorectal cancer. The Wnt/Wg signal leads to the accumulation of beta-catenin/Armadillo in the cell, resulting in the formation of a bipartite transcription complex consisting of beta-catenin and a member of the lymphoid enhancer binding factor/T cell factor (LEF/TCF)HMG box transcription factor family. This complex translocates to the nucleus where it can activate expression of genes downstream of the Wnt/Wg signal, such as the engrailed and Ultrabithorax genes in Drosophila. The downstream target genes of Wnt-1 signaling in vertebrates that presumably function in tumorigenesis, however, are currently unknown.
For a most recent review on Wnt, see Cadigan and Nusse, Genes & Dev., 11: 3286-3305 (1997).
Another family of proteins, the Rho and Rac subfamilies of Ras proteins, have been implicated in transformation by oncogenic ras. Thus far, activation of the pathways governed by three members of the Rho family of GTP-binding proteins, CDC42, Rac, and Rho, has been found to be necessary for Ras transformation. Activating Ras mutations occur in about 30% of all human tumors, indicating that elements of the CDC42, Rac, and Rho signaling pathways are drug targets for cancer therapy. These three members play a central role in the organization of the actin cytoskeleton and regulate transcription. Like Ras, the Rho proteins interact directly with protein kinases, which are likely to serve as downstream effector targets of the activated GTPase. The roles of the different Rho proteins in Ras transformation appear to be distinct: CDC42 specifically controls anchorage-independent growth, whereas Rac controls Rac-induced mitogenicity. The small G proteins Rac1, Rac2, and Rac3 are highly related GTPases. Didsbury et al., J. Biol. Chem., 264: 16378-16382 (1989); Moll et al., Oncogene, 6: 863-866 (1991); Shirsat et al., Oncogene, 5: 769-772 (1990); Haataja et al., J. Biol. Chem., 272: 20384-20388 (1997). RAC3 is located at chromosome 17q23-25, a region frequently deleted in breast cancer. Cropp et al., Proc. Natl. Acad. Sci. USA, 87: 7737-7741 (1990); Cornelis et al., Oncogene, 8: 781-785 (1993). Recent data have provided evidence that constitutive activity of the Rho-family GTPases is associated with cytoskeletal rearrangement and disorganized growth, motility, and invasiveness of cells, all hallmarks of neoplasia.
There is a need to elucidate the further members of the above families, including cell-surface molecules that may be tumor-specific antigens or proteins that serve a regulatory function in initiating the Wnt pathway of tumorigenesis. These would also include downstream components of the Wnt signaling pathway that are important to the transformed phenotype and the development of cancer. There is also a need to identify other proteins that, perhaps in conjunction with beta-catenin, regulate Wnt-1 downstream genes, as well as GTPases.